1 Introduction

The structure-based engineering of ceramic materials has been used more and more widely in aerospace, automotive, electronics, medical, and other fields of the advanced equipment [1]. However, the mechanical processing performance of engineering ceramics is very poor, and the processing tools are required to be higher. At present, the machining removal method of ceramic materials is mainly based on grinding. As a super-hard abrasive, diamond has been used widely in wire drawing dies, dressing tools, drilling tools, sawing tools, etc. [2]. The brazing diamond tool with ordered particles has the advantages of large chip space, high sharpness, and strong holding force with matrix. Therefore, the brazing diamond tool and its processes have become a hot topic of ceramic tool research [3].

In the research of brazing diamond tools, Ni-Cr-B-Si or Ag-Cu-Ti and other alloys are generally used as solder [4]. The brazing temperature of Ni-Cr-B-Si solder is high, the hardness of the formed brazing layer is also high, the brazing temperature of alloy solder such as Ag-Cu-Ti is low, and the hardness of the brazing layer is also lower than the former. As mentioned above, diamond tools are mostly used for processing engineering ceramics and other materials. These materials are very hard and brittle. During the processing process, the brazing layer is susceptible to friction and wear due to high hardness abrasive debris. Therefore, in order to avoid this situation, in actual production, diamond tools mostly use Ni-Cr-B-Si with higher hardness of brazing layer as solder [5,6,7]. So in this paper, Ni-Cr-B-Si is used as a solder for experimental research.

At present, many studies on brazing diamond tools focus on brazing mechanism and solder formulation mainly; however, in terms of quality control of brazing process, there are few studies on the influence of brazing temperature, holding time, solder wetting, and other factors on diamond brazing morphology.

Therefore, this paper prepared brazed diamond wear-resistant blocks with ordered particles by template method [8, 9], and studied the influence of Ni-Cr-B-Si solder on diamond exposing height and wetting angle under different brazing temperatures and holding time, we obtained the optimal brazing temperature and holding time. In addition, the interface of brazing was analyzed by scanning electron microscope (SEM) and energy dispersion X-ray spectroscopy (EDS), reasons of solid metallurgical bond between diamond and solder were analyzed, and the morphology and composition of the interface reaction products were also analyzed.

2 Sample preparation of brazed diamond wear-resistant block with ordered particles

2.1 Experimental materials and equipment

Ni-Cr-B-Si solder, 40-45mesh diamonds, T7 steel plate (used as matrix), flame spraying equipment, vacuum carbon tube furnace, etc.

2.2 Experimental process

Firstly, the surface of T7 steel plate was cleaned, and a 3–5-mm solder layer was formed on the matrix surface by flame spraying [10] and remelting. Then the oxide coating was removed, and diamond particles were orderly arranged on the matrix surface by template method. The prepared soldering complex to be welded was put into a vacuum carbon tube furnace for brazing experiment with a vacuum degree of 2.0 × 10−2 Pa.

The heating process of brazing has two important control parameters, which are brazing temperature and holding time. The higher the brazing temperature, the stronger the fluidity of the solder, the wetting effect of the solder on the diamond will be different, and if the brazing temperature is too high, the degree of diamond carbonization will be exacerbated. The longer the holding time, the more favorable is the strong carbide forming elements in the solder to the interface reaction with diamond. However, if the holding time is too long, it will affect the exposing height of diamond [11]. Therefore, in order to obtain good brazing effect, referring to relevant studies at home and abroad, the experimental brazing temperature in this paper was selected as 1050 °C, 1080 °C, 1110 °C, and 1140 °C, and the holding time was selected as 5 min, 10 min, 15 min, and 20 min. In order to explore the influence of brazing temperatures (1050 °C, 1080 °C, 1110 °C, 1140 °C) and holding time (5 min, 10 min, 15 min, 20 min) on brazing morphology and quality, brazing experiments were conducted on 16 groups of sample blocks. Table 1 lists the control parameter values of each group of samples.

Table 1 Control parameter values of each group of samples
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After the brazing, we observed the surface morphology by microscope. Figure 1 shows the surface topography of brazed tool after brazing when brazing is stable at 1110 °C and holding time is 5 min. As shown in the figure, the template method of distributing diamonds can obtain the wonderful result that the solder is fully melted, and the diamond distribution is uniform and orderly.

Fig. 1
figure 1

Photo of surface morphology of brazing sample

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3 Experimental results

Further observation showed that, after melting, the solder climbed along the diamond surface, and a crater-like shape was formed where the solder contacted the diamond [12].

Figure 2 shows the analysis model of diamond wetted by solder, in which h1 is the total height of the diamond, h2 is the exposing height of the diamond, and the wetting angle of the solder is θ.

Fig. 2
figure 2

Analysis model of diamond wetted by solder

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Sixteen groups of brazing sample blocks with heating parameters were observed by stereomicroscope, and the exposing heights and wetting angles of diamond were measured. There were 8 samples in each group, and the measured mean values were filled in Table 2.

Table 2 Exposing height and wetting angle of brazing diamond
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4 Discussion

4.1 Effect of brazing temperature on wettability of diamond surface

After processing the data from Table 2, the exposing heights of diamond and wetting angles of solder under different brazing temperatures were obtained when the values of holding time were different constants (5, 10, 15, 20 min). The curves of diamond exposure height at different brazing temperatures are shown in Fig. 3.

Fig. 3
figure 3

Exposing height of diamond at different brazing temperatures

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The Fig. 3 shows the exposing height of diamond (h2) decreases with the increase of brazing temperature. When the temperature exceeds the melting point of solder about 150 °C (the solder melting point is 1000 °C), the decreasing trend of diamond exposure height is accelerated.

The Fig. 4 shows the change curves of wetting angle of solder at different brazing temperatures. It can be seen that the wetting angle of diamond decreases with the increase of brazing temperature. When the temperature exceeds the solder melting point about 150 °C, The reduction rate of wetting angle gradually decreases with the temperature increasing.

Fig. 4
figure 4

Wetting angle of solder at different brazing temperatures

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The exposing morphologies of brazing diamond can be directly observed by stereomicroscope, and Fig. 5 shows the diamond exposing morphologies at different brazing temperatures when the holding time is 5 min.

Fig. 5
figure 5

Diamond exposing shapes at different brazing temperatures

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As can be seen from the figure, when the holding time remains unchanged, the exposing height (h2) of diamond presents a trend of gradually decreasing with the increase of brazing temperature. And with the increase of brazing temperature, the wetting angle of solder to diamond decreases, and the wetting effect become better. The reasons are the increased brazing temperatures and the fluidity of solder, so the diamond sinks into the solder easily.

In addition, with the increase of brazing temperature, the interface reaction between diamond and solder is easier, and the element Cr, the strong carbide forming element in solder, is more conducive to diffusion and transfer. Through the contrast analysis, it can be concluded that when the brazing temperature is 1110 °C, the exposing height of diamond, reaching 50% of its own height, is a moderate value, and the solder can provide a higher holding force to diamond, so the particle is not easy to detach from solder.

4.2 Effect of holding time on wettability of diamond surface

The experiment shows that the holding time also has an impact on the brazing effect of diamond. Figure 6 shows the variations of diamond exposing height with different holding time. The exposing height of diamond (h2) increases with the increase of holding time, and the exposing height of diamond (h2) has no sharp change on the whole.

Fig. 6
figure 6

Exposing height of diamond at different holding time

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Figure 7 shows the variations of wetting angle of solder at different holding time. The wetting angle of diamond decreases with increasing holding time, and the variation range of wetting angle decreases when holding time exceeds 10 min.

Fig. 7
figure 7

Wetting angle of solder at different holding time

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Under stereomicroscope, it can be observed directly that the influences of diamond exposing height and solder wetting angle at different temperatures and holding time. As shown in Fig. 8, when brazing temperature is 1110 °C, the diamond exposing morphologies at different holding time can be observed.

Fig. 8
figure 8

Exposing morphology of diamond under different holding time

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Figure 8 shows the surface of diamond has no solder adhesion obviously when the holding time is 5 min. With the extension of holding time, the higher the climbing height of solder on the diamond surface, the more obvious the adhesion phenomenon of solder and the lower the wetting angle significantly. In order to reveal the reason, it is necessary to further analyze the interface. It is found that the carbide created by brazing may cause the decrease of interfacial tension and wetting angle of molten solder.

4.3 Analysis of interface between diamond and solder

In order to further analyze the generating mechanism of diamond morphology under vacuum brazing with Ni-Cr-B-Si solder, the paper selected brazing sample blocks with brazing temperature of 1110 °C and holding time of 5 min and used field emission scanning electron microscope (SIGMA 500/VP) to observe by SEM and to analyze energy spectrum by EDS.

Figure 9 shows the overall morphology of diamond after brazing with Ni-Cr-B-Si brazing solder. It can be seen that the diamond particles are coated with brazing alloy around, and the exposing height is partly maintained. At the interface zone of diamond and solder has a crater-like morphology due to the wetting. As can be seen from the figure, the interface zone shows different changes with the distance between solder and diamond.

Fig. 9
figure 9

Overall morphology of the brazing diamond

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The surface of the diamond far away from the solder is relatively smooth and neat, and the edges of the diamond can be clearly seen. The solder has little thermal damage to the diamond, and the original morphology of the diamond is retained.

Figure 10 shows the topography of the interface resultant between diamond and solder. When the diamond is magnified 3000 times, it is found that the interface resultant near the diamond layer is arranged in fine needle shape, and the closer the spacing with diamond, the finer the resultant. This may be related to the rules of carbon atom arrangement on diamond surface. As the diamond surface is wetted by solder, the interface reaction is more fully performed near the interface between diamond and solder.

Fig. 10
figure 10

Morphology of the resultant at interface zone

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In order to further study the composition of the interface resultant and the distribution of each component in the brazing solder at the interface zone, Fig. 11 shows the energy spectrum line scanning of the local area of interface zone, which covers the part areas of diamond surface and the brazing interface.

Fig. 11
figure 11

Energy spectrum line scanning of the local area

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Cr is a high-strength connecting element and a strong carbide-forming element. Under certain conditions, Cr reacts with C in diamond to generate carbide, and Cr also plays an extremely important role in improving the oxidation resistance, corrosion resistance, and thermal strength of the solder [13]. Figure 12 shows the scanning energy spectrum of brazing diamond interface. It can be seen that the content increases of Cr from the scanning start point at 16 μm. From the point of 20–45 μm, the main components are C and Cr, and the segregation of Cr is obvious. The relevant research on the compounds of C-Cr also proves that the main composition is Cr7C3 in the solder and Cr3C2 at the interface zone between diamond and solder [14,15,16]. Due to the metal carbide in this layer, the interfacial tension between the molten solder and diamond decreases. With the extension of holding time, the content of fine needle C-Cr compounds will increase continuously, which causes the decrease of solder wetting angle. In addition, the carbide increases the holding force strength with matrix, so the particle is not easy to detach from solder.

Fig. 12
figure 12

Scanning energy spectrum figure of brazing diamond interface

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Figure 13 shows the component energy spectrum of interface zone at a given point. By comparing the content of each element in the interface area with the raw brazing solder in Table 3, it can be seen that the content of Fe element is significantly higher than that in the raw brazing solder. This is because when the brazing temperature reaches about 1000 °C, although the matrix is still solid, the Fe element in the matrix has begun to diffuse into the interface zone. The content of Cr element is significantly lower than the average content in solder, which is due to the diffusion of part Cr from solder to matrix.

Fig. 13
figure 13

Component energy spectrum of interface region at a given point

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Table 3 Contents of each element in the raw brazing solder
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5 Conclusion

  1. (1)

    Through brazing experiment of 16 groups of technological parameters(brazing temperature: 1050 °C, 1080 °C, 1110 °C, 1140 °C, holding time: 5 min, 10 min, 15 min, 20 min), we studied the influence of brazing temperature and holding time on the diamond exposing height and solder wetting angle. The results have shown that the diamond exposing height reduces and the solder wetting angle decreases with the brazing temperature increases, but the wetting effect is better. And as the extension of the holding time, the diamond exposing height increases and the wetting angle decreases. In addition, the diamond exposing height is more sensitive to the change of the brazing temperature, and the wetting angle is also sensitive to the change of the holding time.

  2. (2)

    Through observation and measurement analysis of 16 groups of brazing sample, the results have shown that based on Ni-Cr-B-Si solder and 40–45 mesh diamonds, when brazing temperature is 1110 °C and holding time is 5 min, the brazing diamond exposing height is 50% of its own height, and the solder wetting angle is 12°, the brazing effect is the best.

  3. (3)

    Through the observation by SEM and the energy spectrum analysis by EDS of brazing samples, the results have shown that a reaction occurs at the interface between solder and diamond, and the fine needle-like C-Cr compound was created at the interface. Because the interfacial reaction is a strong metallurgical bond, the further wetting and climbing of diamond by the solder was broken.